Skip to main content

Full text of "Observations on a carnivorous snail"

See other formats


SII, OW SII WAI ILOINS) ON AY CUA IRIN OW’ OR 
OUS LAND-SNAIL,. 


By N. ANNANDALE, D.Sc., F.A.S.B., Director, and B. PRASHAD, 
D.Sc., Assistant Superintendent, Zoological Survey of India. 

In the genus Ennea, H. and A. Adams, there are at present 
included a number of species which differ greatly in the complexity 
of the armature of the mouth of the shell. These were separated 
into subgenera by Pfeiffer in Vol. V of his Monographia Helicorum 
Viventium as long ago as 1859, but we have been unable to find 
any detailed description of the armature in any of the more complex 
forms that may be accepted as Enea, s.s.! All descriptions 
of shells of this type that we have been able to find refer merely 
to the external appearance of the aperture and fail to discuss 
the internal structure of the folds and teeth connected therewith. 
As we have recently had an opportunity of examining the com- 
monest and most widely distributed species of the genus both living 
and preserved, we propose to publish here an account of the 
structure of the shell and to add certain observations on other 
points of interest. 


Ennea bicolor (Hutton). 


1908. Ennea bicolor, Blanford and Godwin-Austen, Faun. Brit. Ind., 
Molt. 1, pp. 19, 20, fig. 12. 

For the synonomy and literature of the species see the reference 
cited. Published descriptions of the shell omit several particulars 
of interest in connection both with the aperture and with other 
parts. We give, therefore, as a preliminary to the description of 
a new insular race, a fresh description of the shell of the species. 

The shell is small but somewhat variable in size, with from 
6 to 83 whorls, subcylindrical, with the apex blunt; hyaline and 
almost colourless when fresh but rather thick. The whorls are 
never more than slightly swollen. Their proportions differ some- 
what in different forms of the species. The body-whorl is 
compressed from side to side and bears on each side a funnel-shaped 
depression. That on the inner aspect surrounds the umbilicus, 
which is completely closed. The sculpture consists of very fine, 
straight vertical ribs, which are strongly developed just below the 


1 H.and A. Adams in their original description of the genus, as a stbgenus 
of Pupa, selected no type-species but mentioned Æ. bicolor first on their list, see 
Gen. Recent Mollusca, I1, p. 171 (1858). Whether we accept Æ. bicolor (with 
Blanford and Godwin Austen) as the type-species of the genus, or Æ. elegantior 
(Pfeiffer) with von Martens and Nevill, Huttonella, Pfeiffer is synonymous with 
Ennea, sS 


190 Records of the Indian Museum. [Vor XIX, 


suture on all but the apical whorls and give it a fimbriated or 
almost subspinose appearance. On the greater part of the shell 
these ribs are obsolescent on the lower parts of the whorls, but 
on the whole surface of the outer aspect of the body-whorl they ° 
are well developed. ‘They are absent from the first two and a 
half whorls. The shell as a whole has a smooth and polished 
surface. ‘The aperture is subquadrate but varies somewhat in 
outlines and proportions. It always has the angles rounded. The 
armature consists of two obtuse, somewhat compressed teeth and 
two elongate internal folds. We will describe first its external 
appearance and then its internal structure. The actual orifice is 
couspicuously trilobed owing to the unusually strong develop- 
ment ofthe armature. The three lobes are unequal. The upper- 
most is a narrow sinus lying between the outer lip and a strong 
internal fold, which projects out of the orifice for a short distance 
in the form of a ridge. The second lobe, which is considerably broad- 
er, lies between this ridge and the columellar margin, while the 
third, which is intermediate in size, is bounded externally by a blunt 
tooth lying inside the outer lip at the base of the first lobe. There 
is a second internal tooth near the inner anterior angle of the 
orifice, while a second internal fold lies inside the columellar part 
of the peristome. The peristome itself is thickened and a little 
expanded. It is interrupted by the upper lobe or sinus of the 
orifice, which is pointed and slightly curved, and is often imper- 
fectly developed between the termination of the upper fold and 
the upper extremity of the columella. 

The two folds and the two teeth may now be described in 
detail. The upper fold arises on the floor of the shell about half 
way up the body-whorl and runs down, following the twist of the 
spiral, to emerge from the aperture at the upper extremity a short 
distance within the outer margin. It has the form inside the shell 
of a highly convex crest, but on the edge of the aperture assumes 
that of a low ridge. It is rather thick as a whole and has a blunt, 
but not thickened free edge. The lower fold, although less 
conspicuous externally, is considerably longer and in other respects 
better developed. It arises on the internal column near the suture 
of the body-whorl and runs along the former as a convex crest, 
diverging slightly from the iine parallel to that of the upper fold. The 
free margin is considerably thickened on the inuer side. A broad 
deep gutter, which expands somewhat towards the aperture, is 
thus produced between the two folds. The foot slides along this 
gutter as the animal emerges. ‘The outer tooth represents on the 
internal surface of the shell the external funnel-shaped depression 
on the outer aspect and is thus to a considerable extent hollow. 
It forms with the upper fold a narrow sinus in which the pulmonary 
orifice and the anus lie when the animal is expanded. The inter- 
nal or basal tooth is solid and is not represented by any external 
depression. ‘The depression on the inner aspect of the whorl is 
represented internally by a barely perceptible thickening at the 
outer extremity of the lower fold. 


1920.] N. ANNANDALE & B. PRASHAD : Ennea bicolor (Hutton). 191 


There is preserved in the Indian Museum a series of shells from 
the old A. S. B. collection that Nevill! believed to be ‘‘ probably 
typical specimens from Hutton.” These differ constantly from 
the figure published in the “ Fauna’’ and cited above, in tapering 
less, in having the body-whorl less swollen and in the shape of the 
aperture, which is more elongate and has the columellar and outer 
margin more nearly equal in length. The specimens have the 
sculpture we have described above well developed, though the shell 
has usually been described merely as smooth and polished. 


Ere ante Enis Be 


FIG. 1.—Shell of Exnea bicolor race barkudensis, Annandale & Prashad, 
Ventral view, x 8. 

Fic. 2.—#. bicolor race barkudensis, dorsal view of the last two whorls with 
the shell of the body-whorl removed to show the folds and teeth, 
WO, 
b.t. basal tooth. g gutter, Zf. lower fold. 
o.f, outer tooth, s. sinus, 2.f. upper fold. 


Race barkudensis, nov. 


As we find that individuals from a small island in the Chilka 
Lake differ quite constantly from what we take to be the forma 
typica from the Ganges valley, and alsc from any of the forms 
already described and relegated to the synonomy of the species, we 
propose to describe the race as new under the above name. 

The shell is considerably narrower than that of the forma typica, 
the proportions of breadth to height being 1: 4°5; it tapers less and 
has when mature 84 whorls; the apex is usually a little swollen 
and the aperture is relatively smaller and narrower. The strucutre 
and sculpture of the shell, however, are precisely similar to those 
of the typical form. 


1 Hand List Moll. Ind, Mus. }, p. 6 (1878). 


192 Records of the Indian Museum. Pon AIX, 


Measurements of shells (in millimetres). 


A. B. G: 
Length ne 6:8 6:3 65 
Maximum breadth TO E Te 
Length of the mouth r'5 13 r6 
Breadth of the mouth TA 3 rA 


The animal agrees in colouration with those originally described 
by Hutton? and only differs from Stoliczka’s description? and 
Semper’s coloured figure? in that the searlet colour of the head 
and body is completely restricted to the internal structures con- 
nected with the eye-stalks. The rest of the extruded parts are of a 


G 


JO NAAN beg 


3a. 


3. 


Fic, 3.—Radular teeth of Æ. bicolor race barkudensis, general arrangement. 
3a.—Radular teeth, one side of a complete row of tecth, 
c. central; í, 4, 7, and 9g lateral teeth. 


” 


bright shade of lemon-yellow and the mantle is orange-scarlet. 
We figure the radular teeth. Their shagreen-like arrangement is 
shown in fig. 3. In general facies, doubtless also in function, it 
has an interesting resemblance to that of the papillae on the tongue 
of certain carnivorous mammals. 

Type-specimen. No. M 1131" 
(Ind. Mus.), 

We found this race of E, bicolor fairly common, but by no 
means abundant, on Barkuda Island in the Chilka Take in the 


Zoological Survey of India 


> 


1 Four. As. Soc., Bengal, 1I, pp. 80, 93 (1824). 
2 Tide 25 Tita By peje MeO EN Ait, oll Wilk ites: 7, SS 
8 Reis. d. Philippinen, A11, p. 250, pl. vill, fig. 14 (1870-1804). 


1920.] N. ANNANDALE & B. PRASHAD : Enea bicolor (Hutton). 193 


latter half of June, 1920. The snails were taken on a low flight of 
stone steps leading to a bungalow. They emerged in wet weather, 
and especially on damp evenings, from the interstices of the 
stone-work and the earth at its base. The largest number of 
individuals we saw on any one evening was half a dozen. The 
Ennea was accompanied, in much larger numbers, by an Ofeas, 
which Col. Godwin-Austen ' has identified provisionally as O. graci- 
lis (Hutton). The Ennea was evidently preying upon the Opeas, 
for we found one or two individuals of the former attached to the 
foot of individuals of the Ofeas. The attack, however, was by no 
means vigorous and we could discover very few fresh empty shells. 
Probably this was due to the fact that the carnivorous species 
was just recovering from its hot-weather aestivation. Godwin- 
Austen in the “ Fauna’’ records another instance of the association 
of the two species, observed by Mr. Collet on the sea-wall at 
Galle, Ceylon. Semper also found E. bicolor in the Philippines 
preying on a form of O. gracilis.” 

It would, appear, therefore that these two species (both of which 
are remarkable for their wide distribution and are often found in 
India in places where pot-plants are kept) or closely allied forms 
habitually occur together. On Barkuda the vegetarian snail is, as 
might be expected, very much the more abundant of the two. As 
Godwin-Austen points out in the ‘‘ Fauna,’’ there is a certain 
resemblance between the shells, and this is also so with the animals; 
but they are readily distinguished in life by the scarlet markings 
of the Ennea. ‘These are entirely absent in the Opeas, the mantle 
and exposed parts of which are of a uniform bright yellow colour. 

The resemblance between the gutter produced by the internal 
folds of the shell in Ennea and Clausilia® is very close and must be 
entirely convergent. There is, of course, no clausilium in Ennea 
and the origin of the two main folds is slightly different in the two 
genera. Moreover, supplementary palatal plicae are absent in 
Ennea, as are oral teeth in Clausilia. In other words, the inter- 
nal armature has become exceedingly complex in Clausilia, 
doubtless in correlation with the production of the clausilium, 
while in Enea the external armature is better developed. In the 
latter genus the armature does not suffice to close the shell 
completely when the animal is retracted, but seems rather to 
protect the more important external parts of the body in the act of 
and on the completion of expansion. This may perhaps be useful 
to a carnivorous animal which burrows into soft tissues and 
niticus while feeding, at any rate so far as the upper sinus and the 
pulmonary orifice are concerned. The walls of the gutter between 


l Rec. Ind. Aus. XIU, p. 351 (1917). Col. Godwin-Austen informs us that 
he proposes to describe this form shortly as a new species. 

2 Semper, loc. cit. pp. 137-138, where the species is referred to as Stenogyra 
panayensis; on pl. viii, fig. 15, Semper calls it Subulina panayensis; for syn- 
onym see Gude, Faun. Brit. Ind. Moll. Il, pp. 355, 357 (1914). 

The association of the two species is referred to by Semper on p. 250. 

: anes and Woodward, Ann. Mag. Nat. Hist. (6) V, pp. 209-213, pl. xi, A 
(1890). 


194 Records of the Indian Museum. [VoL. XIX, 1920.] 


the two main folds may also assist in cleaning the foot and head as 
they are retracted. Itis possible, therefore, that whereas, as is 
clearly the case, the whole structure is protective in function in 
Clausilia, in Ennea it assists in active aggression. ‘The case is all 
the more remarkablein that Ennea belongs to a family (the Testa- 
cellidae) in which the shell is often completely degenerate; but it 
is perhaps more common ! than is generally realized to find animals 
of similar habits and related structure adopting in the course of 
evolution diametrically opposite methods of improving their means 
of attack or defence. 


! See Annandale, Four. As. Soc. Bengal (ns.), IN, p. 75 (1913); Mem. 
Ind. Mus. V, p. 54 (1915) and Mem. As. Soc. Bengal, V1, pp. 196-197 (1918) 
and Prashad, Rec. Jud. Afus. XVI, p. 401 (1919). 


aran `~ ~~